We report on successful synthesis under high pressure of a series of polycrystalline GdFeAs O_{1-x}F_x high-Tc superconductors with different oxygen deficiency x=0.12 - 0.16 and also with no fluorine. We have found that the high-pressure synthesis technique is crucial for obtaining almost single-phase superconducting materials: by synthesizing the same compounds with no pressure in ampoules we obtained non-superconducting materials with an admixture of incidental phases. Critical temperature for all the materials was in the range 40 to 53K. The temperature derivative of the critical field dHc2/dT is remarkably high, indicating potentially high value of the second critical field Hc2 ~ 130T.
Single crystals of the unconventional superconductor UTe$_2$ have been grown in various conditions which result in different superconducting transition temperature as well as normal state properties. Stoichiometry of the samples has been characterized by the single-crystal X-ray crystallography and electron microprobe analyses. Superconducting samples are nearly stoichiometric within an experimental error of about 1 \%, while non-superconducting sample significantly deviates from the ideal composition. The superconducting UTe$_2$ showed that the large density of states was partially gapped in …
Its direct momentum sensitivity confers to angle-resolved photoemission spectroscopy (ARPES) a unique perspective in investigating the superconducting gap of multi-band systems. In this review we discuss ARPES studies on the superconducting gap of the high-temperature Fe-based superconductors. We show that while Fermi-surface-driven pairing mechanisms fail to provide a universal scheme for the Fe-based superconductors, theoretical approaches based on short-range interactions lead to a more robust and universal description of superconductivity in these materials. Our findings are also discussed…
We study the behavior of spinless fermions in superconducting state, in which the phases of the superconducting order parameter depend on the direction of the link. We find that the energy of the superconductor depends on the phase differences of the superconducting order parameter. The solutions for the phases corresponding to the energy minimuma, lead to a topological superconducting state with the nontrivial Chern numbers. We focus our quantitative analysis on the properties of topological states of superconductors with different crystalline symmetry and show that the phase transition in th…
Observation of unconventional band topology in a superconducting doped topological insulator, Cux-Bi2Se3: Topological Superconductor or non-Abelian superconductor?
最近发现掺铜的拓扑绝缘体Bi $_2$ Se $_3$在冷却时经历超导转变，这提高了它是第一个已知的“拓扑超导体”或实现新的非阿贝尔超导状态的可能性。它的真正性质取决于关键的体和表面状态带拓扑结构。我们提出了第一个光电子能谱的结果，通过检查在许多不同的铜掺杂值的能带拓扑，我们发现，拓扑保护的自旋螺旋表面状态保持良好的保护和分离的费米能级，其中铜配对发生在最佳掺杂的拓扑绝缘体的体狄拉克带。铜的加入导致非线性电子掺杂和强重整化的拓扑表面态。这些极不寻常的观测结果强烈地表明，在Cu $_x$ Bi $_2$ Se $_3$的拓扑表面上的超导性不能是任何常规类型的一般拓扑理论。首次报道了三维体狄拉克能带结构的超导掺杂态和拓扑不变性质，这将有助于为这种新型超导体制定一个具体的理论。
We measured the low-temperature thermal conductivity of a new layered superconductor with quasi-one-dimensional characteristics, the ternary telluride Ta$_4$Pd$_3$Te$_{16}$ with transition temperature $T_c \approx$ 4.3 K. The significant residual linear term of thermal conductivity in zero magnetic field and its rapid field dependence provide evidences for nodes in the superconducting gap. By measuring resistivity under pressures, we reveal a superconducting dome in the temperature-pressure phase diagram. The existence of gap nodes and superconducting dome suggests unconventional superconducti…
Superconducting Fluctuations and the Reduced Dimensionality of the Organic Superconductor $κ$- (BEDT-TTF)$_{2}$Cu(NCS)$_{2}$ as Observed through Measurements of the de Haas-van Alphen Effect
We analyze a simple model containing the physical ingredients of a Hund's metal, the local spin fluctuations with power-law correlators, $(Ω_0/|Ω|)^γ$, with $γ$ greater than one, interacting with electronic quasiparticles. While the critical temperature and the gap change significantly with varying parameters, the $2Δ_{max}/k_BT_c$ remains close to twice the BCS value in agreement with experimental observations in the iron-based superconductors (FeSC).
Site-dependent NMR Spin-lattice Relaxation in the Superconducting State of an Iron Pnictide Superconductor
在常规超导体中，所有核上的自旋-晶格弛豫速率在低于Tc时应该具有相同的温度依赖性。我们进行了$^{23}$ Na，$^{75}$ As，和$^{59}$ Co NMR研究单晶的NaFe 0.95 Co 0.05 As，并发现自旋晶格弛豫速率显示出非常不同的温度依赖的幂律行为的三个网站。我们认为，这种位置依赖的行为是由于超导体有两个非常不同的大小的差距的事实。每个核的幂律指数受小能隙的超精细耦合强度的影响。我们还发现钴位上的大超导能隙比其它两个位上的小。这表明掺杂剂位置上的超导能隙的局部抑制。
We discuss an analytical model to calculate the superconducting volume ratio. Apart from this, our model can also predict the shape of embedded superconducting domains. We applied our model to calculate the superconducting volume ratios and shape of domains in (TMTSF)$_2$PF$_6$, (TMTSF)$_2$ClO$_4$, YBa$_2$Cu$_4$O$_8$, $β$-(BEDT)TTF$_2$I$_3$ and FeSe. Usually in layered superconductors resistivity drops anisotropically. Our analysis also explains that, this behaviour is due to flat or needle shape of the superconducting samples.
We performed $^{185/187}$Re nuclear quadrupole resonance (NQR) measurements under pressure to investigate the superconducting properties of noncentrosymmetric superconductor Cd$_{2}$Re$_{2}$O$_{7}$ under various crystal structures. The pressure dependence of superconducting transition temperature $T_{\rm c}$ determined through ac susceptibility measurements is consistent with the results of previous resistivity measurements [T. C. Kobayashi $et al$., J. Phys. Soc. Jpn. 80, 023715 (2011).]. Below 2.2 GPa, in the nuclear spin-lattice relaxation rate $1/T_{1}$, a clear coherence peak was observed…
Recent several experiments revealed that novel bipartite magnetic/superconducting phases widely exist in iron pnictides and chalcogenides. Nevertheless, the origin of the two-dome superconducting phases in iron-based compounds still remains unclear. Here we theoretically investigated the electronic structures, magnetic and superconducting properties of three representative iron-based systems, i.e. LaFeAsO1-xHx, LaFeAs1-xPxO and KFe2As2. We found that in addition to the degenerate in-plane anisotropic xz/yz orbitals, the quasi-degenerate in-plane isotropic orbitals drive these systems entering …
Striped superconductors: How the cuprates intertwine spin, charge and superconducting orders
最近的输运实验在原来的铜氧化物高温超导体，{La $_{2-x}$ Ba $_x$ CuO $_4$ }，揭示了一个显着的序列的转变和交叉引起的一种形式的动态降维，其中块体晶体成为基本超导在两个方向上，而它仍然在第三个不良的金属。我们确定这些现象所产生的一个独特的新的超导状态，“条纹超导体，”在超导秩序是空间调制，使其体积平均值为零。在这里，除了概述突出的实验发现，我们勾勒状态的序参量理论，强调条纹超导体从根本上不同于普通（均匀）超导体的一些方式，特别是关于它对猝灭随机性的反应。我们还提出了DMRG计算的结果，在相互作用的电子模型中，超导秩序的符号振荡建立。最后，我们推测这种状态的相关性，在其他铜酸盐的实验，包括最近的光学研究{La $_{2-x}$ Sr $_x$ CuO $_4$ }在磁场中，中子散射实验欠掺杂{YBa $_2$ Cu $_3$ O $_{6+x}$ }，和一系列的STM和ARPES研究中看到的异常{Bi $_2$ Sr $_2$ CaCu $_2$ O $_{8+δ} $ }。
We report superconducting properties of noncentrosymmetric superconductor LaPtGe which crystallizes in noncentrosymmetric $ α$-ThSi$ _{2} $ structure. The magnetization, resistivity and specific heat measurements confirm that LaPtGe is a type-II bulk superconductor with a transition temperature T$ _{c} $ = 3.05 $ \pm $ 0.05 K. Muon-spin relaxation/rotation measurements confirms that time reversal preserved in the superconducting ground state.
The noncentrosymmetric superconductor TaOs has been characterized using x-ray diffraction, resistivity, magnetization, and specific heat measurements. Magnetization and specific heat measurements show a bulk superconducting transition at 2.07 K. These measurements suggest that TaOs is a weakly coupled type-II superconductor. The electronic specific heat in the superconducting state can be explained by the single-gap BCS model, suggesting s-wave superconductivity in TaOs.
Low-temperature thermal conductivity measurements were performed on single crystal of PbTaSe$_2$, a noncentrosymmetric superconductor with topological bulk nodal lines in the electronic band structure. It is found that the residual linear term $κ_0/T$ is negligible in zero magnetic field. Furthermore, the field dependence of $κ_0/T$ exhibits a clear "$S$"-shape curve. These results suggest that PbTaSe$_2$ has multiple nodeless superconducting gaps. Therefore, the spin-triplet state with gap nodes does not play an important role in this noncentrosymmetric superconductor with strong spin-orbital…
